feat: show blocked_count in broadcast admin UI

- Add blocked_count to Broadcast interface
- 4-column stats grid with blocked card (warning color)
- Show blocked count in broadcast list when > 0
- Fetch single broadcast by ID instead of listing all
- Add i18n keys for blocked/blockedShort in ru/en/zh/fa

src/locales/fa.json

@@ -917,6 +917,8 @@
       "total": "کل",
       "sent": "ارسال شده",
       "failed": "ناموفق",
+      "blocked": "مسدود شده",
+      "blockedShort": "مسدود",
       "filter": "فیلتر",
       "message": "پیام",
       "unknownAdmin": "نامعلوم",